The field of the invention is signal transmission using simultaneously several orthogonal (or quasi-orthogonal) carrier frequencies, each coded by distinct data elements.
These signals are generally called OFDM (Orthogonal Frequency Division Multiplex) signals. This type of OFDM signal is used for example in the digital broadcasting system described particularly in French patent FR-86 096322 filed on Jul. 2, 1986, and in the document entitled "Principes de modulation et de codage canal en radio diffusion numeric vers les mobiles" (Principles of channel modulation and coding in digital radio broadcasting to mobiles) (by M. Alard and R. Lassalle; U.E.R. review No. 224, August 1987, pp. 168-190) and known under the name of the COFDM (Coded Orthogonal Frequency Division Multiplex) system.
This COFDM system was developed largely as part of the European DAB (Digital Audio Broadcasting) project. It is digital. More generally, it enables the transmission of any type of digital or analog signal (sampled but not necessarily quantified).
Special demodulators must be used to demodulate these digital signals with frequency multiplexing. For example, this type of demodulator is described in the above mentioned patent document FR-86 09622.
It is known that one essential element of a multicarrier signal receiver is the demodulation circuit which extracts raw information carried by each carrier taken separately, from the received signal (the multiplex of orthogonal carriers).
Conventionally, this circuit carries out mathematical transform the signal, and for example a Discrete Fourier Transform (DFT). Many other transforms may be used. However, this type of circuit will be referred to as a DFT circuit in the following, for non-restrictive simplification purposes.
The complexity of this type of circuit is proportional firstly to the number of frequencies transmitted simultaneously (frequency dimension), and secondly to the duration T.sub.s of transmitted symbols (time dimension). This DFT circuit is a complex and therefore expensive element. Therefore, it is essential that this circuit should be simplified, particularly for low cost receivers.
According to known techniques, the time dimension is limited by reducing the symbol time T.sub.s and/or the guard interval .DELTA. inserted between two consecutive symbols. This limits the number of data processed by the DFT, obviously to the detriment of the received signal quality. As the symbol time increases, the channel selectivity effect is lower, and for a given guard interval sufficient to limit the Inter Symbol Interference to a previously chosen value, the transmitted throughput increases with the length of the symbol time.
In other words, the choice of the DFT size is always a compromise between the received signal quality and the cost price of this DFT.
It is impossible to vary the frequency dimension using known techniques. The DFT must systematically take account of all N carriers forming the transmitted multiplex, even if the information searched by the receiver is distributed only on some of the carrier frequencies.
Conventionally, a transmitted OFDM signal can carry several independent signals. For example in the case of television signals, four distinct signals could be transmitted at 6 Mbit/s on one OFDM signal occupying an 8 MHz band (with a spectral efficiency of 4 bits/s/Hz). Although it would be desirable to recover a single source signal, it is necessary to take account of all carriers at the input to the DFT, which induces complex and partially unnecessary calculations.
Furthermore, in general it is better to use the maximum number of carrier frequencies, particularly to increase the duration of symbols transmitted as described previously.
Once again, we need to find a compromise between the number of carriers and the complexity of the demodulation circuit.